Charged particle accelerators, radiation sources, systems, and methods

ABSTRACT

Man-portable radiation generation sources and systems that may be carried by hand to a site of interest by one or two people, are disclosed. Methods of use of such sources and systems are also disclosed. Battery operated radiation generation sources, air cooled radiation generation sources, and charged particle accelerators, are also disclosed. A radiation generation source with a target less than 0.20 mm is also disclosed.

RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional PatentApplication No. 60/998,691, which was filed on Oct. 12, 2007, and U.S.Provisional Patent Application No. 61/007,500, which was filed on Dec.13, 2007, both of which are assigned to the assignee of the presentapplication and are incorporated by reference herein.

STATEMENT OF GOVERNMENTAL RIGHTS

The U.S. Government has certain rights to this invention pursuant toContract No. H92236-06-D-1004 with the U.S. Department of Defense.

FIELD OF THE INVENTION

This invention relates generally to charged particle accelerators andradiation sources and, more particularly, to lightweight man-portableX-ray radiation sources, systems using such sources, and methods.

BACKGROUND OF THE INVENTION

X-ray scanning has been used to identify explosive materials, such asTNT, and wires of electronic control, timing, and/or detonation devicesfor explosive devices in suspect objects, for example. X-ray scanningmay also be used to identify high atomic number material that may bespecial nuclear materials, such as uranium and plutonium, or shieldingfor such materials, such as tungsten and lead. X-ray scanning may alsoidentify explosive devices that could be used to disperse radioactive,chemical, or biological materials.

Radiation having peak energies of about 0.5 MeV and higher typicallycomprise a particle accelerator, such as a linear radiofrequency (“RF”)particle accelerator, to accelerate charged particles, and a source ofcharged particles, such as an electron gun, to inject charged particlesinto the accelerator. The linear accelerator may comprise a series oflinearly arranged, electromagnetically coupled resonant cavities inwhich standing or traveling electromagnetic waves for accelerating thecharged particles are supported. The charged particles injected into theresonant cavities are accelerated up to a desired energy and directedtoward a conversion target to produce radiation. Where the acceleratedcharged particles are electrons and the target is a heavy material, suchas tungsten, Bremsstrahlung or X-ray radiation is generated. Electronsaccelerated to a nominal energy of 1 MeV and impacting tungsten, willcause generation of X-ray radiation having a peak energy of 1 MeV, forexample.

A microwave (RF) power source provides RF power to the cavities of theaccelerator. The microwave source may be an oscillating microwave powertube, such as a magnetron, or an amplifying microwave power tube, suchas a klystron. The microwave sources are powered by modulators, whichgenerate high electric power pulses having peak electric powers of from1 MW to 10 MW, and average powers of from 1 kW to 40 kW, for example.

Typical MeV radiation sources weigh several tons. Once set up at alocation for scanning, they are not readily moved. Portable MeVradiation sources are known, which can be moved by truck or forklift,for example. They may be more readily moved to different locations.

One example of a portable MeV radiation source is a Mini-Linatron, whichwas available from Varian Associates, Palo Alto, Calif. As described inliterature from Varian Associates, the Mini-Linatron comprised an X-rayhead, a power module, a control case, and a modulator module that wereconnectable by transmission lines, cables and hoses. The X-ray head,which is said to have weighed from 100 pounds (45 kg) including a 2 MV-6MV accelerator, and 300 pounds (136 kg) including a 9 MV accelerator,also contained an ion chamber and a collimator. The power module, whichis said to have weighed 300 pounds (136 kg), contained a magnetron, apulse transformer, and other RF components. The modulator module, whichweighed 300 pounds (136 kg), is also said to have contained a pulsemodulator, an electronic line-type chassis, and a power supply. Thecontrol case is said to have weighed 11 pounds (5 kg). A Mini-Linatronincluding a 2 MV accelerator therefore weighed about 711 pounds (323kg). “MINI Field Portable X-Ray Equipment,” Varian Associates, October1997.

Another modular high energy source for mobile and fixed installations,available from Varian Medical Systems, Inc., Palo Alto, Calif.(“Varian”), is the Linatron®-M™. An X-ray head module including a 3 MVM3 Linatron® accelerator and RF unit, which includes a magnetron andpulse transformer, is said to weigh 1,950 pounds (886.3 kg).“Linatron®-M™ Modular high every radiation source,” Varian MedicalSystems, Inc., September 2007.

Another portable system which was available from Varian is theLinatron-MP, in which an X-ray head module including a 4 MV acceleratorweighs 150 pounds (68 kg), a modulator cabinet weighs 685 pounds (311kg), and an RF unit weighs 340 pounds (155 kg). “VARIAN'S LINATRON-MP:THE PORTABLE SYSTEM” for Field Radiography,” Varian Medical SystemsTechnology, Inc., 2003.

Russell G. Schonberg describes a 4 MeV traveling wave acceleratorpackaged with a 9.3 GHz magnetron r.f. source and a pulse transformer,weighing about 190 pounds (86 kg), in “A History of the Portable LinearAccelerator.” Schonberg states that “the total weight was marginal fortwo people at 190 pounds . . . ” Schonberg does not identify the weightof the modulator and power supplies, which, as described above,typically weigh many hundreds of pounds. “The History of the PortableLinear Accelerator”, Russell G. Schonberg, The American Association ofPhysicists in Medicine, Annual Meeting, 2001.

SUMMARY OF THE INVENTION

As used herein, the term “man-portable radiation source” means aradiation source with components that are arranged in subunits that maybe carried by one or two people to a site of interest and set up, ascompared to a “portable” radiation source, which has been used to referto a source that is non-permanent and relocatable or movable by aforklift, a dolly, rolling on integral wheels, or lifting by multiplepersons. Similarly, a “man-portable radiation scanning system” means aradiation scanning system with components that are arranged in sub-unitsthat may be carried by one or two people to a site of interest and setup.

In accordance with an embodiment of the invention, a man-portableradiation generation system is disclosed comprising a first modulecontaining at least one battery and a second module containing amodulator. The first and second modules are configured to be selectivelyelectrically coupled to each other. The system further comprises a thirdmodule containing a charged particle accelerator. The second and thirdmodules are configured to be selectively electrically coupled to eachother and the at least one battery provides power to the first andsecond module when the first, second, and third modules are electricallycoupled. Each module is portable by hand by one or two people. Eachmodule may be portable by hand by one person. The system may weigh lessthan 300 pounds (136 kg), or less than 225 pounds (102 kg), for example.The first, second, and third modules may each weigh less than 100 pounds(34 kg) or less than 75 pounds (34 kg), and at least one of the first,second, and third modules may weigh less than 50 pounds (23 kg). Atleast one of the modules comprises a case with handles.

The first module may further comprises a controller to control operationof the source and a control device removably mounted to the firstmodule, for remote control of the controller. The first module mayfurther comprise a cable electrically coupling the control device to thecontroller, and a spool, around which the cable is selectively wound. Anelectrical plug may be provided for connection to an external powersource.

The third module may further comprise an electron gun mounted to theaccelerator, to inject electrons into the accelerator, a target coupledto the accelerator to generate X-ray radiation upon impact byaccelerated electrons, a magnetron coupled to the accelerator to provideradiofrequency power to the accelerator, and the modulator, which inthis example is powered by the at least one battery, provides power tothe electron gun and to the magnetron. The third module may weigh lessthan 80 pounds (36 kg).

The third module may also comprise a rigid support coupled to at leastone inner wall of the third module, and the accelerator may be coupledto the support. The support is coupled to the at least one inner wall byat least one resilient member. The accelerator and the magnetron may besuspended from the support, at a position such that respective spacesare provided between the accelerator and the magnetron, and an opposingwall of the case. The support may comprise a rigid plate connected tothe at least one inner wall and at least one elastomeric member couplingthe accelerator to the rigid plate. A second rigid plate may be coupledto the first rigid plate by the at least one elastomeric member and theaccelerator may be connected to the second rigid plate.

The portable radiation generation system may be configured to generateradiation having a peak energy of about 1.0 MeV, for example. The systemmay be configured to generate radiation greater than 500 kHz and lessthan about 1 MeV, for example.

A plurality of fins may be coupled to an exterior surface of theaccelerator. At least some of the plurality of fins are transverse to along axis of the accelerator. A cover covering at least some of theplurality of fins may be provided, to form a cooling manifold having afirst opening for air to enter the cooling manifold and a second openingfor air to exit the cooling manifold. The third module has at least onewall defining at least one air inlet opening. At least one fan isproximate the at least one air inlet vent to move air through the thirdmodule and a guide is provided to direct air into the first opening. Aduct may be provided to convey air from the at least one air inlet ventto the guide. A duct may convey air from the fan to the first opening.

In accordance with another embodiment, a man-portable radiation scanningsystem is disclosed comprising the man-portable radiation sourcedescribed above and a detector. The system may further comprise adisplay to be coupled to the detector array. The detector may compriseradiographic film, for example.

In accordance with another embodiment of the invention, a man-portableradiation generation source is disclosed comprising a first modulecomprising a case containing at least one battery and a second modulecomprising a second case containing a source of charged particles, acharged particle accelerator, a target, a modulator, and a magnetron,wherein the first and second modules are configured to be selectivelyelectrically coupled to each other. Each module is portable by hand byone or two people.

In accordance with another embodiment, a charged particle accelerator isdisclosed comprising a source of charged particles and an acceleratorcomprising a buncher cell defining a buncher cell cavity. The chargedparticle source is coupled to the buncher cell to inject electrons intothe buncher cell cavity, which captures and r.f. focuses the injectedelectrons into an electron beam. A plurality of linearly arranged cellsdefining periodic, linearly arranged accelerating cavities aredownstream of the buncher cell, to receive and accelerate the electronbeam. An output cell is downstream of the accelerating cells, to receiveand output the accelerated electron beam. The cells further define aplurality of linearly arranged on-axis coupling cavities betweenrespective cells. The buncher cell and a first periodic cell followingthe buncher cell are configured such that a field step ratio between thepeak amplitude of the electric field in the first cell cavity and thepeak amplitude of the electric field in the buncher cell cavity isgreater than one (1), during operation. A cell period ratio between acell length from a center of one periodic cell cavity to a center ofnext accelerator cell cavity, and half the free space wavelength of theaccelerator during operation, is less than one (1). The field step ratiomay be less than (2), during operation. The field step ratio may be from1.2 to 1.5, or from 1.3 to 1.4. The cell period ratio may be greaterthan 0.78 and less than 0.82. The field step ratio may be 1.3, the celllength may be 12.5 mm, and the cell period ratio may be 0.78. A bunchercell ratio between a length of the buncher cell and half the free spacewavelength of the accelerator may be less than one-half. The bunchercell ratio may be 0.3. The accelerator may comprise periodic couplingcavities between the periodic accelerating cavities.

The accelerator may be configured to define a particle beam having aspot size encompassing 75% of the beam on the target having a diameterof less than 2 mm, during operation. The accelerator may weigh sevenpounds (3.2 kg) or less. As described above, a plurality of fins may becoupled to an exterior wall of the accelerator, and a cover may beprovided to cover at least some of the plurality of fins to define acooling manifold with openings for air to enter and exit the coolingmanifold. A tube adjacent to an outer wall of the accelerator may beprovided to provide cooling or heating fluid adjacent to the outer wall.

A magnetron may be coupled to the accelerator. The magnetron may drivethe accelerator with radiofrequency energy having a frequency selectedto excite the resonate cells with standing waves with π/2 radian phasebetween a coupling cell and a next accelerating cell. The magnetron maydrive the accelerator at a frequency of 9.3 GHz, during operation, forexample. The charged particle source may comprise an electron gunconfigured to operate at the same voltage as the magnetron, or lowervoltage. The electron gun may comprise an anode plate coupled to thebuncher cell. The buncher cell may define a half-cell and the bunchercell cavity may be defined by the half-cell and the anode plate. Theanode plate may define an aperture with an entrance to the bunchercavity dimensioned to remove charged particles at a periphery of thecharged particle beam. The diameter of the entrance may be dimensionedto remove at least half of the charged particles in the beam.

The accelerator may comprise ten periodic accelerator cavities betweenthe buncher cavity and the output cavity, for example. A target may becoupled to the output cell, wherein impact of charged particles on thetarget generates radiation.

In accordance with another embodiment, a radiation generation source isdisclosed comprising a linear charged particle accelerator, a source ofcharged particles coupled to the accelerator to inject charged particlesinto the accelerator, and a target coupled to an output of theaccelerator. Impact of the accelerated charged particles on the targetcauses generation of radiation. A plurality of fins are coupled to anexterior surface of the accelerator, to air cool the accelerator, asdescribed above. At least some of the plurality of fins may betransverse to a long axis of the accelerator and a cover covering atleast some of the plurality of fins to define a cooling manifold havingopenings for air to enter and exit the cooling manifold may also beprovided. The accelerator may be contained within a case with at leastone wall defining an air inlet. A fan may be proximate the inlet tocause air to move through the case and a guide may direct air into thefirst opening, during operation. The guide may be coupled to themanifold, and a duct may convey air from the fan to the guide. A ductmay convey air from the fan to the first opening, instead. The at leastone wall of the case may further define at least one exhaust vent.

In accordance with another embodiment, a method of setting up aman-portable radiation source to examine an item of interest isdisclosed, wherein the radiation source comprises at least a firstmodule containing at least one battery to power the source and a second,separate module comprising an accelerator, an electron gun, and atarget. The method comprises carrying by hand the at least first andsecond modules to a location proximate the item of interest, by at leastone person, electrically coupling at least the first module to thesecond module by at least one electrical cable, and moving a safedistance from the radiation source, leaving the at least first andsecond modules at the site. The method may further comprise removing acontrol device from one of the modules and moving to the safe distance,with the control device, leaving the at least first and second modulesat the site. The control device may be electrically coupled to a cablerolled around a spool in the one module and the method may furthercomprise unrolling a spool of cable in the one module and moving to thesafe distance with the control device. The method may comprise moving toa safe distance, behind a dense structure. The method may furthercomprise activating the source to generate radiation and scanning theitem of interest with the generated radiation. A detector may be carriedby hand to the location and radiation interacting with the item ofinterest may be detected by the detector. A third module containing amodulator may be carried to the location and electrically coupled to thefirst and second modules.

In accordance with another embodiment, a battery operated radiationgeneration source is disclosed comprising at least one battery, acharged particle accelerator, a source of charged particles coupled tothe accelerator to inject charged particles into the accelerator, and atarget coupled to an output of the accelerator. Impact of theaccelerated charged particles on the target causes generation ofradiation. A radiofrequency power supply provides radiofrequency powerto the accelerator. The at least one battery provides power to thesource of charged particles and the radiofrequency power supply. Amodulator may be coupled to the at least one battery, to convert directcurrent voltage from the battery to pulses of high voltage to beprovided to the source of charged particles and to the radio frequencypower supply.

In accordance with another embodiment, a radiation generation source isdisclosed comprising an electron source and an accelerator comprising abuncher cell defining a buncher cell cavity. The electron source iscoupled to the buncher cell to inject electrons into the buncher cellcavity and the buncher cell cavity captures and r.f. focuses theelectrons injected by the electron source, into an electron beam. Aplurality of linearly arranged cells define periodic, linearly arrangedaccelerating cavities downstream of the buncher cells to receive andaccelerate the electrons. An output cell is downstream of theaccelerating cells and a target is coupled to the output cell to receiveand output the accelerated electron beam. A target is coupled to theoutput cell. Impact of accelerated electrons on the target causesgeneration of X-ray radiation. The buncher cell, the accelerating cells,and the output cell further define a plurality of linearly arrangedon-axis coupling cavities between respective cells. The buncher cell anda first periodic cell following the buncher cell are configured suchthat a field step ratio between the peak amplitude of the electric fieldin the first cell cavity and the peak amplitude of the electric field inthe buncher cell cavity is greater than one (1), during operation. Acell period ratio between a distance between from a center of oneperiodic cell to a center of next accelerator cell, and half the freespace and length of the accelerator during operation, is less than one(1). The field step ratio may be less than two (2), during operation. Abuncher cell ratio between a length of the buncher cell and half thefree space wavelength of the accelerator may be less than one-half. Thebuncher cell ratio may be 0.3. The output cell may define an inwardlytapered passage from a cavity to a target. The may comprise a coppersubstrate and a tungsten layer coupled to the copper substrate. Thethickness of the tungsten layer may be less than 0.25 mm, less than 0.20mm, less than 0.10 mm, or less than 0.05 mm. Other features describedabove may be incorporated in the accelerator in accordance with thisembodiment of the invention, as described in more detail in thespecification.

In accordance with another embodiment, a radiation generation source isdisclosed comprising a charged particle accelerator, a source of chargedparticles coupled to the accelerator to inject charged particles intothe accelerator, and a target coupled to an output of the accelerator.Impact of the accelerated charged particles on the target causesgeneration of radiation. The thickness of the tungsten layer is lessthan 0.20 mm. The thickness of the tungsten layer may be less than 0.10mm. The thickness may be 0.05 mm.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic block diagram of an example of a man-portableradiation source in accordance with one embodiment of the invention;

FIG. 2 is a perspective view of an example of a case for any or all ofthe modules;

FIG. 3 is an example of a man-portable radiation system incorporatingthe man-portable radiation source of FIG. 1, at a site of interest;

FIG. 4 is a perspective view of an example of an accelerator inaccordance with an embodiment of the invention;

FIG. 5 is an axial sectional view of the accelerator of FIG. 4,excluding the electron gun and target assembly, to simplifyillustration;

FIG. 6 a is an enlarged sectional view of a buncher half-cell of FIG. 4;

FIG. 6 b is an enlarged sectional view of a half-cell connected to thebuncher half-cell, of FIG. 4;

FIG. 7 is an enlarged sectional view of a half-cell of FIG. 4;

FIG. 8 a is a perspective view of an example of a target assembly foruse with the accelerator of FIGS. 4 and 5;

FIG. 8 b is a sectional view of the target assembly of FIG. 8 a, throughline 8 b-8 b;

FIG. 9 is a perspective view of another example of an accelerator withguides coupled to respective cooling fin assemblies;

FIG. 10 a is a perspective view of an assembly comprising the magnetronand the accelerator, coupled to a strong back;

FIG. 10 b is another example of the assembly supported by another strongback;

FIG. 11 is a graph of energy E (arbitrary) versus Z (cm) for theaccelerator;

FIG. 12 is a sectional view of an example of the internal configurationof the X-ray head in the third module, including another strong backarrangement; and

FIG. 13 is a sectional view of another example of the internalconfiguration of the X-ray head in the third module, including anotherstrong back arrangement.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

There may be times when it would be advantageous to quickly and easilyset up an X-ray scanning system at a particular site by one or twopeople. For example, the ability to quickly and easily deploy alightweight radiation source for object examination by one or two peoplecould facilitate the identification of explosive devices hidden insuspect objects at crime scenes, actual or potential sites of terroristattacks, or in combat or war-time situations. Hidden improvisedexplosive devices (“IEDs”) may thereby be identified, for example. Sucha lightweight radiation source could also facilitate the identificationof flaws and faults in infrastructure, such as bridges, as well as theexamination of small or difficult to access locations, such as in anairplane or submarine, for example. A radiation source and radiationscanning system including such a source, which may be carried to a siteby one or two people, would therefore be advantageous.

FIG. 1 is a schematic block diagram of an example of a man-portableradiation source 100 in accordance with one embodiment of the invention.As used herein, the term “man-portable radiation source” means aradiation source with components that are arranged in subunits that maybe carried by one or two people to a site of interest and set up, ascompared to a “portable” radiation source, which has been used to referto a source that is non-permanent and relocatable or movable by aforklift, a dolly, rolling on integral wheels, or lifting by multiplepersons. A man-portable radiation source may be used in a “man-portableradiation scanning system,” which, as used herein, means a radiationscanning system with components that are arranged in sub-units that maybe carried by one or two people to a site of interest and set up.

In this example, the man-portable source 100 is designed to generate anX-ray radiation beam having peak energy of about 1 MeV (1 MeV+/−10%). Ina particular example described herein, the peak energy is 0.93-0.94 MeVand the generated X-ray radiation has a half value layer (“HVL”) of 0.57inches (14.5 mm)-0.62 inches (15.7 mm). The HVL is the length of steelrequired to reduce X-ray dose or intensity by half. The man-portableradiation scanning system 100 in one example may image an 18 gauge (7mm) diameter copper wire through 3 inches (7.6 cm) of steel. Asdiscussed above, electronic control, timing, and/or detonationelectronics for explosive devices may include wires. These are justexemplary energies and higher energy (greater than about 1 MeV)man-portable radiation sources may be made in accordance withembodiments of the invention at other energies and HVLs. For example,man-portable radiation sources of 3 MeV or 6 MeV may also be provided.In addition, lower energy radiation sources, such as 500 KeV sources andhigher may also be made in accordance with embodiments of the invention.

In this example, the X-ray source 100 comprises separate first, second,and third modules 102, 104, 106, respectively, each light enough to becarried by one or two persons. In this example, each module 102, 104,106 weighs less than 100 lbs (45 kg). Certain modules may weigh lessthan 75 lbs (34 kg) or less than 50 lbs (23 kg), for example. Eachmodule 102, 104, 106 and thereby the source 100 are thereforeman-portable, meaning that each module may be moved by one or two peoplewithout the assistance of a machine, such as a forklift.

The first module 102 in this example comprises a controller 108, one ormore batteries 110, and a remote control (or pendant) 112. The secondmodule 104 in this example comprises a modulator 114. The third module106 in this example comprises an X-ray head 118. The X-ray head 118 inthis example comprises an electron gun 120, an accelerator 122, a target124, a magnetron 126, and a pulse transformer 128. The first module 102may be coupled to the second module 104 by a first, control cable 120and a second, power cable 122. The second module 112 may be coupled tothe third module 106 by a control cable 124 and a drive cable 130. Thedrive cable may comprise two separate cables, one for the filaments ofthe electron gun 120 and magnetron 126, and another for pulses providedto the electron gun and magnetron. The controller 108 controls operationof the source 100, under the control of the pendant 112, which is aportable remote control that may be physically mounted in the firstmodule 102 when not in use. The batteries 110 provide DC power to themodulator 114, which converts the DC power to pulses to drive themagnetron 126 and electron gun 120. The pulse transformer 128 permitsuse of a lower voltage on the cable connectors. The magnetron 126generates an electromagnetic field that is provided to resonant cavitieswithin the accelerator 122. Electromagnetic standing waves are supportedwithin the accelerator 122. Electrons provided by the electron gun 120to the accelerator 122 are accelerated by the standing electromagneticwaves. The accelerated electrons impact the target 124 causinggeneration of X-ray radiation by the Bremsstrahlung effect.Alternatively, a traveling wave accelerator may be used.

FIG. 2 is a perspective view of an example of a case 170 for any or allof the modules 102, 104, 106 when resting on a surface. Two handles 171are shown along different sides of the case 170. Additional handles on aside or a longer handle may be provided to facilitate carrying by twopeople, if desired. Two exhaust air vents 172 are provided, one showncovered by a precipitation deflector 174 a in FIG. 2 and the other shownwith the deflector 172 b separated from the case 170 for illustrativepurposes. FIG. 2 a shows a view of the inner surface of the deflector174. Returning to FIG. 2, two inlet air vents 176 are also provided, onecovered by a deflector 174 c and the other shown with the deflector 174d separated from the case 170 for illustrative purposes. Intake fans 178are provided in the inlet air vents 176 to increase air flow through thevents and the case 170, as described below. In this example, each case170 for each of the modules 102, 104, 106 is identical, to decrease costand for simplicity, but that is not required. An EMI shielded filterscreen 180, shown in FIG. 2 b, may be provided in the inlet vents 176and the exhaust vents 172, as well.

The case 170 may be a commercially available case or a custom designedcase. The case 170 shown in FIG. 2 is a commercially available StormCase IM 2950, available from Hardigg Industries, South Deerfield, Mass.,which weighs 20.8 lbs. (9.4 kg) (without foam). The internal dimensionsof the IM 2950 are 29 inches (74 cm)×18 inches (46 cm)×10.5 inches (27cm). The Storm Case IM 2590 does not include openings for vents. Theinlet vents 176 a, 176 b and the exhaust vents 172 are therefore added.The Storm Case IM 2950 includes wheels, which may be removed to furtherdecrease the weight of the modules, if desired.

Another commercially available case 170 is the Pelican 1650(R),available from Pelican™ Products Inc., Torrance, Calif., which weighsabout 29.1 lbs (13 kg), (without foam). The Pelican 1650(R) has internaldimensions of 28.5 inches (73 cm)×17.37 inches (44 cm)×105 inches (266cm). The Pelican 1650(R) also does not include openings for vents andthe inlet vents 176 a, 176 b and the exhaust vents 172 would need to beadded. As above, wheels may be removed, if desired.

Another commercially available case 170, which is lighter than thePelican 1650(R), is the Seahorse SE 1220, available from Seahorse,Covina, Calif., which weighs about 24.44 lbs (11 kg). The internaldimensions of the SE 1229 are 25.52 inches (65 cm)×19.5 inches (50cm)×13.08 inches (33 cm). The Seahorse SE 1220 also does not includeopenings for vents and the inlet vents 176 a, 176 b, and the exhaustvents 172 would need to be added. As above, wheels may be removed, ifdesired.

To inspect an item of interest 150 with the man-portable X-ray source100 in accordance with one embodiment of the invention, the modules 102,104, 106 may be driven to a site near the item of interest by a vehicle,such as a car, jeep or truck, for example, and unloaded. The modules102, 104, 106 may then be carried to and positioned proximate the item150 by one or two people, as shown in FIG. 3. One person may carry oneor two of the modules 102, 104, 106 by the handles 140, at a time. Iftwo handles 140 on a side or a long handle are provided, as discussedabove, two people can carry one module at a time.

To assemble the source 100 proximate the item of interest 150, the thirdmodule 106 is positioned a distance P from the item, as shown in FIG. 3,for example. The distance P may be any suitable distance. For example,the distance P may be about 1 meter. The first and second modules 102,104 are positioned near the third module 106 and are coupled to eachother via a control/power cable 120/122, which may be a combined cableor separate cables. The second module 104 is coupled to the third module106 by a combined or single control/drive cable 130/132. The position ofthe first and second modules with respect to the third module may dependon the length of the cables 120/122, 130/132. In this example, thecables 120/122, 130/132 are about 1 meter long, to reduce capacitiveeffects and still allow for flexibility in placement of the modules 102,104, 106 at the scanning site. In one example, the man-portable X-raysource 100 may be assembled in about two minutes, for example.

One or more detectors 160 may be positioned a suitable distance behindthe item of interest 150 to detect radiation transmitted through theitem, as shown in FIG. 3, and/or one or more detectors 160 may bepositioned to detect scattered radiation. The detector 160 may be animaging panel, such as the HE4030 imager system, available from VarianMedical Systems, Inc., Palo Alto, Calif., for example, which weighsabout 30 lbs (14 kg), or X-ray film. Film packets may weigh from about10 lbs (4.5 kg) to about 12 lbs (5.4 kg), for example. Other types ofdetectors may be used, instead.

A processor 162 may be coupled to the detector 160 by a cable 163, and adisplay 164 may be coupled to the processor. The processor 162 and thedisplay 164 may comprise a laptop computer weighing about 5 lbs (2-3 kg)to about 20 lbs (9 kg), depending on the model, for example. Thedetector 160, the processor 162, and the display 164 may be carried tothe site by one or two people in one or two trips. In this example, thecable is long enough for the processor 160 and the display 164 to beused by an operator about 30 m from the X-ray source, as discussedbelow, such as from about 30 m to about 40 m, for example The detector160 may be coupled to the processor 162 wirelessly, if interference isnot a concern.

The detector 160 may also comprise X-ray film, in which case a processorand display are not needed. Since film developers are quite large andheavy, a film developer is not incorporated in the system. The film maybe carried from the site to a developer in another location. Imaging ofan 18-gauge wire through 3 inches (76 mm) of steel could take from about3 minutes of X-ray beam-time for some film types up to about one hourfor others. Use of film requires shielding of unexposed film from theradiation field emitted by the X-ray head 118, as is known in the art,to protect against premature exposure by radiation from the unshielded,non-collimated X-ray head 118. Commercially available film, such asGAFCHROMIC® EBT film, available from International Specialty. Products,Wayne, N.J., may be used, for example.

Other components as needed may also be carried to the site and coupledto the modules 102, 104, and/or 106. For example, the accelerator 122,the modulator 114, and/or the magnetron 126 may require water coolingand/or heating, as discussed below. A water supply and pump (not shown)may be carried to the site and coupled to the modulator 114, theaccelerator 122, and/or to the magnetron 126. The water supply and pumpmay be included within one of the modules, such as the first module 102,or in a separate module, for example.

In the example described herein, no collimator is used and radiation Ris emitted in all directions. Alternatively, the third, X-ray headmodule 106, may include a recess (not shown) to receive a fielddeployable collimator, which could weigh about 30 lbs (14 kg). Thecollimator may be carried to the site of interest separately from themodules 102, 104, 106, for example.

A major source of the weight in a radiation source is radiationshielding. To reduce the weight of the third module 106, no shielding isprovided around the X-ray head 118. However, radiation is emitted in alldirections, which could increase the risk of deleterious exposure tooperators and others in the area. In the example described herein,radiation leakage may be as high as 1 R/m at 1 meter, or 60,000 mR/hour.

To protect the operator and other personnel from dangerous radiationexposure, after assembly of the X-ray source 100 at a site, distance isused to reduce exposure along with field expedient measures, ifavailable, such as taking cover behind a masonry wall, an earthen berm,or other dense object. Personnel should move as far as possible from thethird module 106. They should be at least about 30 meters from the thirdmodule 106, in this example. At 30 meters in the open, the X-ray dosecould be as high as 70 mR/hour at 1 meter, corresponding to a “radiationarea.”

An operator may remove the pendant 112 from the first module and carrythe pendant to the safe location and/or behind a dense structure, suchas a concrete wall or building, if in the vicinity. Such a densestructure may provide sufficient protection at less than 30 meters fromthe third module 106. If the pendant 112 is connected to the controller104 by the cable 130, the cable may be wound on a spool 132 when storedin the first module 102 and unwound as the operator moves to the safelocation. The cable may be 40 meters long, for example. If wirelesslycontrolled, the operator may similarly move to the safe location beforeactivating the system 100.

Personnel assembling and operating the source 100 may also carrypersonal dosimeters to monitor their exposure. An alarm bell may beprovided to alert the personnel to a pre-set exposure. Perimeter accessshould be controlled to avoid exposure to others. Separate shieldingslugs of shielding material (not shown) may be positioned around thethird module 106 at a desired site, if desired, to enable personnel tobe closer to the item of interest 150 during operation.

Where a digital imager is employed, the image may be analyzedimmediately on a laptop, from the operator position. As discussed above,the digital imager may be an HE4030 imager from Varian Medical Systems,Inc., for example. It takes about 3-seconds for the HE4030 imager togenerate an image. Since beam on time is reduced when using a digitalimager, radiation exposure of personnel may be reduced by a factor ofabout 50 as compared to the use of film. Digital imaging also reducesbattery usage, increasing battery life compared to use of film. TheHE4030 digital imager, which weighs about 15 lbs, may be stored in thefirst module 102. A laptop computer may be used to process and displaythe images, as is known in the art.

After imaging of the item of interest 150, the modules 102, 104, 106 andthe X-ray source 100 may be quickly disconnected and removed from thesite, by one or two people carrying each module 102, 104, 106.

The First Module

As discussed above, the first module 102 contains the controller 108,the batteries 110, and the pendant 112, and related components. Thecontroller 108, which controls operation of the source 100, may be aprocessor, such as a programmable logic controller (“PLC”), which may bea commercial off the shelf processor board. A battery operated,“wireless” radiation source, which is not limited to use nearconventional power supplies, is more versatile than a source that mustbe plugged in to a conventional source. As discussed below, however, theman-portable radiation source 100 may be driven by conventional AC powerin addition to or instead of batteries 110.

As discussed above, the pendant 112 is a portable remote control thatmay be physically mounted in the first module 102 when not in use. Inone example, a display screen is provided to display status information,such as warming up, beam on, exposure time and/or dose, and remainingbattery life, for example. The pendant 112 may be coupled to thecontroller 104 by a cable 130 or where the application does not havesensitive electronics, wirelessly. If wirelessly controlled,electromagnetic/radio-frequency interference may need to be controlled.The cable 130 may be wound on a spool 132. As discussed above, a 40meter cable may be used, for example. The pendant 112, controller 104,and cables 120, 122, 130, 132 may weigh up to about 10 lbs (4.5 kg), forexample.

Functions on the pendant 112 may include a red emergency off button tode-energize the system 10, a yellow warning light for a fault causingthe X-ray beam to turn off, and a manual override button to provide aninstant “beam-on” and “beam-off”, for example. Alternatively, thependant 112 may be mounted in any of the other modules 104, 106 wherethere is room. If the pendant 112 and the controller 108 are indifferent modules, additional cables may be required. The third module106 may include an emergency off button instead or in addition to theemergency off button on the pendant 112.

The batteries need to supply sufficient power to image for a desiredperiod of time. In one example, the batteries provide sufficient powerto scan for about 100 minutes continuously, at about 1 Rad/minute. Inorder to supply such power for such a period of time, the typical powerrequirements and operating levels of other components of the source,such as the modulator 114, the electron gun 120, and the magnetron 128,need to be conserved. That requires changes in the typical design of theaccelerator 122, examples of which are described below.

The batteries 110 in the first module 102 generate DC voltage, such as240 volts, which is provided to the modulator 116 in the second module112 via the power cable 122. In one example, the batteries need to store640 kilojoules. The batteries 106 may comprise a pack of ten (10) 24volt commercial batteries, for example. The battery pack may weigh about20-25 pounds (9 kg-11 kg), for example. A separate compartment in thefirst module 102 may be provided for the cables 120, 122, 130, 132 andthe pendant 110. The batteries may be rechargeable and/or replaceable inthe field.

The batteries may be a BA 5590 lithium/sulphur dioxide battery packsystem from Saft Groupe SA, Bagnolet, France (“Saft”), which is said tocomprise 10 LO26 SX cells connected in two groups of 5 cells in series,providing 2 nominal 12 volt sections at the connector, for example. Thesections may be connected in series to provide 24 volts or in parallelto provide 12 volts. According to a specification provided by Saft, thetypical operating control voltage (“OCV”) is 15.0 or 30.0 volts, thenominal voltage (at 500 mA) is 13.5 or 27.0 volts, and the cutoffvoltage is 10.0 or 12.0 volts, depending on whether the sections areconnected in series or in parallel. The typical capacity (at 70° F. (21°C.)), 250 mA discharge current is said to be 15 hours in a 12 volt modeand 24 hours in a 24 volt mode. The batteries are said to operate over atemperature range of from −40° F. (40° C.) to 160° F. (71° C.). Eachbattery is said to weigh 2.25 pounds (1 kg), and the battery pack weighsabout 22.5 pounds (10 kg).

Alternatively, lithium ion polymer batteries, such as LIP-5 (“LIP”)available from LINCAD, Ltd., Camberley, Surrey, England may also beused, for example. Lithium ion rechargeable batteries, such as theUBI-2590, available from Ultralife Batteries, Inc., Newark, N.J., forexample, may also be used. Nickel metal hydride rechargeable batteries,such as those used in battery operated cars, may also be used.

An electrical plug 128 and cable 129 may be provided in the first module102 for connection to a conventional source of AC power, such as a walloutlet providing 110 volts or a generator, for example. If the batteries106 are rechargeable, the AC power may be used to recharge thebatteries. The modulator 114 may also be powered by an AC power source(not shown) during use, if the cable 129 is long enough to reach it.

A fan (not shown) may be provided for further air circulation andcooling.

The weight of the first module 102 in this example is from about 50 lbs(23 kg) to about 80 lbs (36 kg), depending on the weight of the case170. If the Storm Case IM 2950 is used, for example, the first module102 would weigh about 50 lbs (23 kg) to about 70 lbs (32 kg), forexample, which may be readily carried by one person.

The Second Module

The second module 104 contains the modulator 114, which converts the DCpower provided by the batteries 106 to suitable pulses to drive themagnetron 126 in the X-ray head 118 in the third module 106, as is knownin the art. In one example, the modulator 116 converts the 24 voltsprovided by the batteries 106 to 2.2-2.4 microsecond pulses at about 29kilovolts and 30 Amps. Alternatively, the modulator 104 may be includedin the same module 106 as the X-ray head 118. While increasing theweight of the third module 106, fewer cables would be required,decreasing the risk of arcing.

To reduce the weight of the source 100, the X-ray head 118 in thisexample is designed to operate under less power than typical X-ray heads(as discussed below), allowing for a smaller modulator 114. With theX-ray head 118 described in this example, a 29 kV, 30 A modulator may beused. In this example, the modulator 114 is a commercially availablemodulator weighing about 75 lbs (34 kg) or less.

For example, the Stangenes Model SSM-3-3-M1, available from StangenesIndustries, Inc., Palo Alto, Calif., may be used. The SSM-3-3-M1, whichweighs about 75 lbs (34 kg), is capable of 36 kV, 80 A at 0.001 dutywith a 2-millisecond pulse.

Alternatively, the Scandinova Model Type M1, which also weighs about 75lbs (34 kg), provides 48 kV and 110 A at 0.0012 duty, available fromScandinova A B, Uppsala, Sweden, may be used.

As in the first module 102, the case 170 housing the second module 104includes vents and one or more fans (not shown).

The weight of the second module 106 is about 75 lbs (34 kg) plus theweight of the case 170. If the Storm Case IM 2950 is used, the secondmodule would weigh about 96 lbs (44 kg), for example, which may becarried by one or two people.

The Third Module

As discussed above and shown in FIG. 1, the third module 106 containsthe X-ray head 118. The X-ray head 118 comprises an electron gun 120, anaccelerator 122, a target 124, a magnetron 126, and a pulse transformer128.

FIG. 4 is a perspective view of an example of an accelerator 1000 inaccordance with an embodiment of the invention. The accelerator 1000comprises a biperiodic, standing wave electron beam linear acceleratorbody 1002. The accelerator 1000 operates in the X-band at 9.3 GHz.X-band accelerators may be smaller than S-band accelerators, whichoperate at 3 GHz, as is known in the art. An S-band accelerator may beused in accordance with embodiments of the invention, if a larger andheavier X-ray radiation source 100 may be tolerated. An electron gun1004 is coupled to one end of the accelerator body 1002 and a targetassembly 1006 is coupled to the opposite end. The electron gun 1004 iscoupled to the accelerator body 1002 via an anode plate 1008. Awaveguide window 1010 and a waveguide 1012 couple the accelerator body1002 to the magnetron 126 (shown in FIG. 9 a). In this example, thewaveguide window 1010 defines a rectangular opening 1010 a. A vacuumpump 1013 is coupled to the waveguide 1012 to create a vacuum within thewaveguide and the accelerator body 1002. An optional cooling tube 1014for water cooling or heating of the accelerator body 1002, is alsoshown. Cooling fins 1016, may also be provided instead of or along withthe cooling tube 1014, as discussed in more detail below.

The accelerator body 1002 shown in the example of FIG. 4 weighs fromabout 6 pounds (2.7 kg) to about 7 pounds (3.2 kg). The accelerator 1000has a length “L” of about 6 inches (about 15 cm) not including theelectron gun 1004 but including the target assembly 1006. Theaccelerator body 1002 has an outer diameter of about 35 mm without thecooling fins 1016 and about 96 mm with the fins. The dimensions of thecooling fins 1016 are based on providing stable operation over anambient temperature range of from about 0° C. to about 56° C. The fins1016 have been found to provide stable operation up to about 70° C.ambient. Smaller fins 1016 may be used if operating conditions are moretightly controlled. The accelerator 1000 would then have a smallerdiameter.

Electron guns for many X-ray radiation sources are typically driven at ahigh voltage of about 20 kV to about 100 kV with a separate power supplyor transformer. Higher voltage requires larger clearances (10 kV/inch,254 kV/mm), and also adds to power supply weight. To reduce the weightof the X-ray head 118, the accelerator 1000 is designed to allowoperation of the electron gun 1004 at about the same voltage as themagnetron 126, or less voltage. The accelerator 1000, in this example,also accommodates a low accelerating gradient, which may be 6 MV/M, forexample, required by the relatively low peak power available from themodulator 114 and the magnetron 126. The electron gun 1004 is driven ata lower than typical voltage of 26 kV-29 kV.

The electron gun 1004 may be a commercially available diode gun with aperveance of 0.1 uperv. The electron gun voltage is at or below themagnetron voltage, which in this example is 28 kV. Voltage is providedto the electron gun 1004 via a high voltage connector 1004 a.

The vacuum pump 1013 may be a 0.2 liter/second ion pump, referred to asa Vacion pump, such as a mini ion pump with smaller magnets, Part Number8130038, available from Varian Vacuum Technologies, Torino, Italy, forexample.

FIG. 5 is an axial sectional view of the accelerator 1000 of FIG. 4,excluding the electron gun 1004 and target assembly 1006, to simplifyillustration. In this example, the accelerator body 1002 comprises achain of cells 1020-1042 defining respective electrically coupledresonant accelerating cell cavities 1020 a-1042 a. The first cell 1020is a buncher cell, which defines a buncher cell cavity 1020 a configuredto bunch and focus the injected electrons to form a beam and toestablish its size. Buncher cells are generally described in U.S. Pat.No. 6,864,633, for example, which is assigned to the assignee of thepresent invention and is incorporated by reference herein. Ten (10)full, in-line, periodic electrically coupled resonant accelerating cells1022-1040 follow the buncher cell 1020 in this example. The term“periodic” as used herein means that the accelerating cavities 1022a-1040 a defined by each respective cell 1022-1040 have the samedimensions. The waveguide 1012, which couples the magnetron 126 to theaccelerator body 1002, is coupled to the sixth full accelerating cell1032, in this example. The final cell 1042 defines an output cavity 1042a, from which accelerated electrons exit the accelerator body 1002.

The buncher cell cavity 1020 a is defined by the anode plate 1005 andthe buncher cell 1020, which is a half-cell. The anode plate 1005defines an output of the electron gun 1004, which in this example tapersto a narrow aperture 1056. The aperture 1056 is inwardly tapered towardthe buncher cell cavity 1020 a, in this example, and may have andiameter of 0.0050 inch (0.13 mm), for example. Such a small diameterfacilitates a rapid creation of the electromagnetic field in the bunchercell. The small aperture 1056 has also been found to “scrape” off theouter electrons in the electron beam, reducing the electron beam currentand diameter. About half of the electrons may thereby be removed. Thisreduces the peak power requirements of the accelerator 122 andintroduces a smaller diameter electron beam to the buncher cell 1020.Enlarging the diameter of the aperture 1056 to 0.080-0.100 inches (0.2mm-2.5 mm) provides higher current and better transmission. If such alarger aperture 1056 is used, the buncher field step (discussed below)may need to be adjusted.

The buncher half-cell 1020 includes an iris or opening 1054. Thecross-section of the buncher half-cell 1020 is shown enlarged in FIG. 6a. A shallow cavity 1055 is provided on an opposite side of the buncherhalf-cell 1020 a as the cavity 1020 a. The iris 1054 electrically andphysically couples the cavities 1020 a, 1055, allowing for the passageof RF energy and an electron beam, as is discussed further below. Thecavity 1020 a of the buncher half-cell 1020 faces the anode plate 1005.The buncher half-cell 1020 is partially received within a recess 1005 ain the anode plate 1005.

In this example, the maximum diameter D1 of the buncher cell cavity is26.71 mm; the diameter D2 of the iris 1054 is 6.52 mm; the maximumdiameter D3 of the coupling cavity 1055 is 26.65 mm; the depth De1 ofthe buncher cell cavity 106 is 3.32 mm; the depth of De2 of the couplingcavity is 0.49 mm; the depth De3 of the iris of 1052 is 1.0 mm; and thelength L_(b) of the buncher cell 1053 is 4.81 mm.

Each half-cell 1060 includes a first, deep cavity 1062, a beam tunneliris or opening 1064, and a second, shallow cavity 1066 on an oppositeside of the half-cell 1060 of the first, deep cavity 1062 and facing anopposite direction, as shown in FIG. 7. The full accelerating cavities1022 a are formed by identical facing cup shaped half-cells 1060, one ofwhich is shown enlarged and in cross-section in FIG. 6 b, and another ofwhich is shown in FIG. 7. The shallow cavity 1055 of the buncher cell1020 is attached to the shallow cavity 1066 a of the first half-cell1060 a of the first resonant cell 1022 to form a full coupling cavity1055, as shown in FIG. 6 b. As shown in FIG. 5, the half-cells 180 arejoined such that a first, deep cavity of one cell faces a first, deepcavity of an adjacent facing cell and a second, shallow cavity of onecell faces a second, shallow cavity of another adjacent cell. Thematching larger cavities form the full cells 1022-1040 and acceleratingcavities 1022 a-1040 a, while the matching shallow, second cavities formthe coupling cavities 1070-1088. The irises 1064 and the couplingcavities 1070-1088 electrically and physically couples the cavities1062, 1064, allowing for the passage of RF energy and an electron beam,as is discussed further below.

In this example, each half-cell 1060 defines a deep cavity 1062 having amaximum diameter D4 of 27.07 mm and a cavity depth De4 of 4.78 mm; aniris 1064 having a diameter D5 of 6.44 mm and an iris depth De6 of 0.49mm; and a coupling cavity 1066 having a maximum diameter D6 of 26.65 mmand a cavity depth De5 of 0.49 mm.

The irises 1054, 1064 of the buncher cell 1020, the accelerating cells1022-1040, and the output cell 1042, are aligned with the axis X of theaperture 1056 of the electron gun 1008 to form a tunnel for passage ofan axial electron beam (not shown) through the accelerator body 1002, asshown in FIG. 5. The full resonant cells accelerate the electronsinjected by the electron gun while the coupling cells 1070-1088electrically couple the accelerating cavities 1022 a-1040 a to eachother. The sum of the accelerations in each cavity 1020 a-1042 a add inthe aggregate to the desired energy of 0.93 MeV-0.94 MeV, in thisexample.

The output end 1065 of the accelerator body 1002 is defined by a fullcell 1042, which is formed in this example by another half-cell 1060 anda larger, deeper half-cell 1062 facing the half-cell 1060. For example,the half-cell 1060 may have a depth of about 4.78 mm and the deeperhalf-cell 1062 may have a depth of about 7.39 mm. A tapered passage 1064extends from the half-cell 1062 to the target assembly 1006, which iscoupled to the output end 1065. The tapered passage 1064 is dimensionedto intercept outlying electrons where cavity tuning will be lessaffected by heat.

The target assembly 1006 (not shown in this view) fits within the recess1062 a. FIG. 8 a is a perspective view of an example of the targetassembly 1006. FIG. 8 b is a sectional view of the target assembly 1006of FIG. 8 a, through line 8 b-8 b. In this example, the target assembly1006 comprises a copper substrate 1072 supporting a tungsten button 1074in a cavity 1072 a. The tungsten button 1074 is brazed to the coppersubstrate by a copper/gold braze 1076. The braze 1076 may comprise 35%copper/65% gold, for example. Grooves 1078 may be provided in the coppersubstrate 1072 through which gas is pumped to create a vacuum and avoida virtual leak, as is known in the art. In one example, the tungstenbutton 1074 is 2 thousandths of an inch (0.05 mm) thick and has adiameter of 0.3 inches (7.6 mm). Usually, tungsten target buttons are 10thousandths of an inch thick (0.25 mm). It has been found, however, thata tungsten target button with a thickness of less than 10 thousandths ofan inch (0.25 mm) provides higher radiation yield. For example,progressively better yield may be obtained with button thicknesses ofless than 0.20 mm, 0.15 mm, and 0.10 mm, such as 0.05 mm. In thisexample, use of a tungsten button 1074 with a thickness of 0.05 mmincreased the yield by about 50% compared to a tungsten button of 0.25mm. The higher yield increases the radiation dose, enabling fasterimaging. This is advantageous, especially where the total imaging timemay be limited due to battery capacity. The braze is 1-2 thousandths ofan inch thick (0.025-0.05 mm). The target button 1074 and othercomponents of the target assembly 1070 may comprise other materials,instead of or in addition to those noted here, as is known in the art.The target assembly 1070 may also be mounted on a ceramic spacer toprovide electrical insulation and to permit monitoring of targetcurrent, as is known in the art.

While it is common in accelerators for the cell cavity lengths toincrease from cell to cell, in this example, the cell cavity length iskept the same, except in the buncher cell cavity 1020 a and the outputcell cavity 1042 a. This facilitates manufacture and assembly of thehalf-cells, since only one size half-cell 1060 is needed (besides thebuncher cell 1020 and output cell 1042). All the half-cells 1060 aretherefore interchangeable. However, cell lengths may be varied, ifdesired.

As shown in FIGS. 4 and 5, an optional cooling and/or heating tube 1014extends along portions of the exterior surface of the accelerator body1002. If such a cooling tube 1014 is to be used, then a water pump maybe set up next to the third module 108 at the site and coupled to thecooling tube, as discussed above. The water pump could weight about 100lbs (45 kg) or less, which may be provided in a fourth module, or thefirst module 102, if desired. The cooling tube 1014 may be made ofcopper and have an outer diameter of ⅜ inch (9.52 mm) and a wallthickness of 0.065 inch (1.65 mm). The pump may pump water at a rate of1 to 2 l/s, at 20° C.-40° C., for example. The cooling and/or heatingtube 1014 may be used for testing of the accelerator 1000, as well.

Instead of or in addition to the cooling tube 1014, cooling fins 1016may be provided around the accelerator body 1002 for cooling. In FIG. 4,two rear cooling fin assemblies 1150 a, 1150 b are shown. Two forwardcooling fin assemblies 1150 c, 1150 d are shown in part, in phantom. Inthis example, each assembly comprises fourteen fins 1016 brazed to theaccelerator body 1002 forming 13 ducts for air passage. The fins 1016 ineach assembly 1150 a, 1150 b, 1150 c, 1150 d are covered by a respectivesolid outer casing 1154. The fins 1016 are separated by a distance of0.3 cm in this example. Each fin has an inner diameter of 36 mm, and anouter diameter of 96 mm. Each assembly may extend 120° around theaccelerator body 1002, for example.

One or more fans may be provided in the third module 106, to draw airinto and through the third module 106, over the cooling fins 1016. Asdiscussed above with respect to FIG. 2, two inlet vents 176 a, 176 b,each containing a fan 178 may be provided. One or more guides may becoupled to or adjacent to the cooling fin assemblies 1150 a, 1150 b,1150 c, 1150 d to guide air drawn into the third module 108, across thefins 1016, as discussed below with respect to FIG. 9. Each fin 1014 maybe made from 0.015 inch (0.38 mm) thick copper sheet. The fins 1014 maybe assembled into the assemblies 1150 a-1150 d with a copper/gold brazeand brazed to the accelerator body 1002 by a copper/silicon (Cusil)braze. The total weight of the four fin assemblies 1150 a-1150 d isabout 1 lb (0.45 kg). Fins may be arranged longitudinally along theaccelerator 1002, instead.

FIG. 9 is a perspective view of another example of an accelerator 1000,with respective guides 1151 coupled to respective cooling fin assemblies1150 a, 1150 d, 1150 c. The guide coupled to the cooling fin assembly1150 b is not visible in this view. Air drawn into the third module 108enters a first, open end of each guide 1151 along the arrows and exitsthe guides into the ducts between the fins 1016 at the lower portions ofthe cooling fin assemblies 1150 a, 1150 b, 1150 c, 1150 d. The air exitsthe upper portions of the ducts at the top of the cooling fin assemblies1150 a, 1150 b, 1150 c, 1150 d, carrying away heat from the fins 1016and accelerator 1002. Air drawn into the third module 106 by one or morefans 178 may flow into the guides 1151. Alternatively, the guides 1151may be coupled to a fan or fans by ducting. In one example, a 3.5 inch(9 cm), 120 CFM fan draws air into a duct with a four way splitter. Fourducts extend from the splitter, one to each guide 1151. An example of aduct 1152 connected to a fan 178 in a vent in a wall 106 a of the thirdmodule 106 is shown in phantom, coupled to one of the guides 1151. It isnoted that in FIG. 9, the vacuum pump 1013 is rotated 90° with respectto the orientation of the pump 1013 in FIG. 4, to accommodate the guides1151. FIG. 9 also shows a support 1153 bolted to the waveguide 1007 andthe anode plate 1005, to support the anode plate.

Louvers and/or vents may be provided on the third module 106 foradditional cooling along with or instead the cooling tube 1014 and/orthe cooling fins 1016. The third module 106 may also comprise resistiveheaters, if needed, for use in cold environments. Louvers and/or ventsmay also be used for heating in cold environments.

The magnetron 126 in this example, which provides microwave power to theresonant cells within the accelerator 1002, is a modular, X-band (9.3kHz) magnetron, with a motor activated mechanical tuner to adjustfrequency, and filament leads powered to heat the cathode surface,permitting microwave emission. X-band magnetrons used with X-bandaccelerators generating X-ray radiation typically generate a power of1-1.5 MW. To reduce weight in this example, the accelerator 1002 isdesigned to accelerate electrons to the desired energy (in this example0.93-0.94 MV, 1 rad/min) with a lower power magnetron 126. In thisexample, the magnetron 126 generates a peak output of less than 400 KW,an average power of 200 W, at a duty cycle of 0.0005.

The power of the magnetron 126 in this example is about 340 KW, at avoltage of 28 KV and a current of 29 Amps. Due to losses in thewaveguide 1012, the peak power at the accelerator 1002 is less thanabout 320 KW and the average output power is less than about 200 W. Themagnetron 126 may weigh about 10 pounds (4.5 kg), for example.

The accelerator 1002 is designed to operate at about 290 KW, providing awide margin that has been found to avoid the need for mechanical tuning.Prior art accelerators typically require mechanical tuning or polishingof cells to establish accurate an resonant frequency plane. Tuning maybe provided in any particular configuration, if needed.

FIG. 10 a is a perspective view of an assembly 1200 comprising themagnetron 126 coupled to the accelerator 1000. A circulator 1210, whichcontrols the flow of microwave fields, is coupled to the magnetron 126by an E-plane bend. The circulator 1210 is coupled to the acceleratorbody 1002 through a length of waveguide 1220 that is coupled to thewaveguide window 1010 shown in FIG. 4, through a second E-plane bend. Adry load 1230 is coupled to the circulator 1210 through an H-plane bend.The dry load 1230 absorbs reflected waves from the circulator 1210, asis known in the art. The circulator 1210 may be an Isolator RF System, 3GHz, 240 kWp, 120 Wavg, WR 112, circulator from Advanced FerriteTechnologies, Germany, Part No. 1-0930020503, for example, which weighsabout 5 lbs (2 kg).

In this example, the magnetron 126, circulator 1210, accelerator body1002, and associated components are coupled to a support or “strongback” 1240 of a rigid, light weight metallic or composite material, suchas aluminum, by four brackets 1262, 1264, 1266, and 1268. Elastomericisolators 1250, such as metallic or plastic springs or elastomericmaterial, for example, are also provided to isolate vibrations when theassembly 1200 is mounted in the third module 106. Suitable elastomericisolators may be obtained from Lord Corporation, Cary, N.C. For example,206 steel multiplane platform mounts, Part Number 206P-45, may be used.According to Lord Corporation, these platform mounts, which comprises aninner portion of specially compounded rubber and an outer portion ofcold rolled steel, have a maximum axial rated load of 3/16 inch (4.80mm) deflection of 45 lbs (200 N), and an axial spring rate of 240 lbs/in(42.0 N/mm). FIG. 10 b is another example of an assembly 1240 supportedby another strong back configuration, in which three brackets 1262,1266, and 1268 connect the accelerator 1000 magnetron 126 and associatedcomponents to the strong back 1240. Elastomeric isolators 1250 are alsoshown, having a different configuration than those in FIG. 10 a. Thestrong back 1240 and isolators 1250 may weigh from about 3 lbs (1.36 kg)to about 10 lbs (4.5 kg) in total, for example.

The magnetron 126 may be a VMX 3045 magnetron available from CPI BeverlyMicrowave Division, Beverly, Mass. According to Company specifications,the VMX 3045 weighs 9.9 lbs (4.5 kg), has a rated maximum output of 380kW and is operable at a duty factor of 0.0005.

Other commercially available magnetrons that may be used include the VMX1131 Magnetron available from CPI Beverly Microwave Division, Beverly,Mass. According to company specifications, the VMX 1131 has a rated peakoutput of 325 kW, and typical performance at a level of 400 KW. It issaid to be rated an X-band coaxial magnetron operating over a frequencyof 8.5 GHz-9.6 GHz. It is also said to be rated at a duty cycle of 0.001and 3.5 milliseconds, an anode voltage of 29 KV, an anode current of 30A, and a 9 volt heater with a power output of 14 A. It is air-cooled bya fan and is mechanically tunable. The VMX 1131 requires 30 A, 29 KV at320 KW and weighs 17 lbs (7.7 kg). The VMX 1131 is said to be operableafter 3 minutes warm-up at air into a matched load in the temperaturerange of from about −55° C. to about 270° C., 40 cfm air. It has beenfound to operate into an accelerator at about 5 to about 30 psi SF₆.

The magnetron 126 may also be a CalTube PM-1100X, a CalTube PM-1000X, ora CalTube PM-325X, provided by CalTube Labs, a unit of L3 CommunicationsApplied Technologies, Watsonville, Calif., for example, which weigh 35lbs (16 kg). According to specifications provided by CalTube Labs, theCalTube PM-1100X is rated at 1.5 MW peak output at 36 kV and 80 A, with0.001 duty cycle. It is tunable over +/−25 MHz, employs an integralpermanent magnet. It requires a nominal 0.66 gpm water cooling. Asdiscussed above, a water pump may be set up proximate to the thirdmodule 106 if needed. It has a 300-second warm-up time. Also accordingto specifications provided by CalTube Labs, the CalTube PM-1000X israted at 1.2 MW for 32 kV-80 A and 0.0007 duty cycle; and the CalTubePM-325X provides 325 kW peak power with 28 kV-35 A at 0.001 duty cycle.

The pulse transformer 128, shown schematically in FIG. 1, is coupled tothe electron gun 1004 and the magnetron 126, permitting use of a lowervoltage on the cable connectors, improving their reliability anddurability. A suitable pulse transformer 128, weighing about 10 lbs (4.5kg), is available from Stangenes Industries, Inc., Palo Alto, Calif.,for example.

The power of the magnetron 126 and the electron gun 1004 may beselectively varied in this example to vary the dose rate of theradiation beam from about 0 to about 2 rads/min, at 1 meter, a depth ofdose maximum (dmax). The power may be controlled by the controller 108under the control of the pendant 110, for example.

The weight of the X-ray head 118 in this example is from about 35 lbs(16 kg) to about 55 lbs (25 kg) or 60 lbs (27 kg). If the Storm Case IM2950 170 is used, for example, the third module 106 would weigh fromabout 55 lbs (25 kg) to about 75 lbs (34 kg) or 80 lbs (36 kg), whichmay be readily carried by one or two people.

A man-portable radiation scanning source 100 in this example wouldtherefore weight from about 200 lbs (91 kg) to about 250 lbs (113 kg),depending on the case 170. A man-portable radiation scanning system 100including such a man-portable radiation source 100 and a digital imagermay therefore weigh from about 235 lbs (107 kg) to about 300 lbs (136kg).

In another example, only two modules are provided, the first module 102and a second module containing the modulator 114 and the X-ray head 118.In this example, the second module could weigh from about 125 lbs (57kg) to about 150 lbs (68 kg), which may be carried by two people to thesite. Total system weight could be reduced by eliminating one case 107and the cables necessary to couple the second module 104 to the thirdmodule 106. Placing the modulator 114 closer to the X-ray head 118 alsoreduces power losses along long cables.

Operation

In operation, microwave energy generated by the magnetron 126 isprovided to the cavities 1020 a-1042 a of the accelerator body 1002, viathe rectangular opening 1010 a of the waveguide 1010, which in thisexample is coupled to the sixth accelerating cavity 1032 a. (See FIGS. 4and 5).

The microwave energy propagates through the accelerator body 1002, fromone cavity 1020 a-1042 a to the next, through the coupling cells1055-1088, setting up alternating positive and negative portions ofstanding electromagnetic waves in the buncher cell 1020, the fullaccelerating cell cavities 1022 a-1040 a, and the output cell cavity1042 a. The standing waves pass through zero in each coupling cell1055-1088. A high voltage pulse is applied to the electron gun 1004 bythe modulator 114 in the second module 104 via a high voltage connector1004 a, as is known in the art.

The aperture 1056 focuses electrons from the electron gun 120 as theyenter the buncher cell 1020. The electrons are accelerated by the timevarying electromagnetic standing waves in the buncher cell cavity 1020a. Since the electrons are only accelerated half the time in the fieldin the buncher cell cavity 1020 a, the electrons “bunch.” The phase atwhich they bunch, the capture fraction, and the radial focusing of theelectrons are determined by the cell geometry, which is discussed inmore detail, below. The electron beam converges as it enters the bunchercell 1020. As the beam diverges within the buncher cell cavity 1020 a,it receives a focusing “kick” by radial forces generated by the standingelectromagnetic waves in the buncher cell cavity 1020 a. The beam thenpasses through the iris 1064 into the first full cell cavity 1022 a,where it diverges again. Radial forces of the standing electromagneticwaves in the first cell cavity 1022 a again focus the beam. The beam isalso accelerated by longitudinal forces caused by the standingelectromagnetic waves in the cell. The electron beam then passes throughthe downstream iris 1064 of the first full resonant cell 1022 into thesecond full cell cavity 1024 a. The diverging of the beam, the focusingof the beam, and the acceleration of the beam are repeated in eachsubsequent accelerating cavity 1022 a-1040 a, and the output cavity 1042a.

The phase of acceleration need not be perfect in the buncher cell cavity1020 a, the first cell cavity 1022 a, and the subsequent cell cavities1024 a-1042 a. Instead, in one embodiment, the phase is optimized suchthat, for equal length cells, the net phase-error over the length of theaccelerator body 1002 is minimized and the spectrum is thereby narrowed,providing for efficient conversion of microwave energy into X-rays. Inthe known prior art, in contrast, phase optimization is attempted ineach cell cavity. This typically requires a multiplicity of uniqueparts, including a plurality of different sized cells, which increasesdesign complexity and cost. Such accelerators may also be more sensitiveto manufacturing and operating parameters. As discussed below, thestructure of the parameters of the accelerator 1000 are adjusted toprovide stable operations with low sensitivity to manufacturing andoperating parameters. It is noted that focusing is achieved in thisexample without an external solenoid, reducing the size and weight ofthe accelerator 1000. An external solenoid may be provided, however, ifthe additional size and weight of the accelerator 122 may be tolerated.

The standing waves accelerate the electrons as the electrons passthrough each cell cavity 1022 a-1042 a. The acceleration per cell cavityand number of cell cavities are arranged to provide the electrons withthe desired peak acceleration. In this example, the cell cavities 1022a-1042 a accelerate the electrons to the desired 0.93 MeV-0.94 MeV.Since low power is used to reduce the size and weight of the modulator114 and the X-ray head 118, including the accelerator 1002 and themagnetron 126, the electrons in the electron beam are acceleratedslowly. In this example, ten (10) full accelerating cells 1022 a-1040 aare required to accelerate the electrons to the desired energy.

The accelerated electrons exit the accelerator body 1002 through theoutput cell 1042 and the passage 1064, toward the tungsten button 1074in the target assembly 1006. Impact of the accelerated electrons withthe tungsten button 1074 generates radiation having a peak energy ofabout 0.93 MeV-0.94 MeV, by the Bremsstrahlung effect. Unlesscollimated, the generated radiation beam will be emitted from thetungsten button 1074 and out of the third module 108 in all directions.

The half value layer (“HVL”), which is the length of steel required toreduce an X-ray dose or intensity by one-half, is an indication of theenergy of the X-ray beam and the quality of the X-ray spectrum. In thisexample, the X-ray radiation generated by the radiation source has anHVL (“HVL”) of from about 0.57 inches (14.5 mm) to about 0.62 inches(15.7 mm) with power peaking the spectrum at about 0.9 MV. Operating at250 Hz and pulse-width of 2 us, for a duty cycle of 0.0005, thedose-rate output in a 10 cm×10 cm field at 1 m, with probe at d_(max) insolid-water, is in the range of 1 R/m. An 18 gauge (7 mm diameter)copper wire may be imaged through 3 inches (7.6 cm) of steel, with awide variety of commercially available X-ray film, as well as a digitalpanel.

The HVL is affected by the “quality” of the X-ray spectrum, which refersto the spread of the energy spectrum. To achieve this HVL with theman-portable X-ray source 100 in this example, the electron beam has arelatively narrow energy spectrum. In this example, 40% of the electronsin the electron beam lie within 6% of the peak acceleration energy of0.93 MV-0.94 MV.

A second figure of merit used to quantify the operation of anaccelerator is the mean of the energy E^(n) raised to the 1/nth power(<E^(n)>^(l/n)), where E is the energy in MV and n=2.7, compared to thepeak energy in the spectrum. This value determines the X-ray doseoutput, according to yield Y=0.07 I_(avg)E^(2.7), where I_(avg) is theaverage current in micro-amps, and the yield is expressed inRad/min/microamp. This value has also been found to correlate well withthe HVL figure for the X-ray beam, which is also an aggregate measure.Depending on the operating power of the magnetron 126 and the electrongun 1004, this figure is over 0.64 MV or 72% of the peak. Consideringthe size, weight, and power constraints on the accelerator 1002, this isa very “tight” radiation beam.

Another factor affecting the HVL is spot size of the electron beam onthe target. In this example, the spot size of the radiation beam, whichencompasses 75% of the electron beam on the target, has a diameter ofless than 2 mm.

Three ratios related to the structure and operation of the accelerator122 also contribute to achieving the desired HVL and spot size in thisexample. One is the ratio α between the peak amplitude of the field inthe first full cell cavity 1022 a to the peak amplitude of the field inthe buncher cell cavity 1020, referred to herein as the “field stepratio,” the second is the ratio between the length L_(b) of the bunchercell cavity 1020 a and half the free space wavelength (λ/2), referred toherein as the “buncher cell ratio,” and the third is the ratio betweenthe cell cavity period L_(p) and half the free space wavelength (λ/2),referred to herein as the “cell period ratio.”

The field step ratio α affects the balance between focusing anddefocusing of the electron beam from the buncher cell 1020 to the firstcell 1022. The field step ratio α also affects the phase of theelectrons exposed to the standing electromagnetic fields in thedownstream cells. In one example, the peak field ratio is greater thanone (1) and less than two (2). For example, the peak field in thebuncher cell may be about 70% of the peak field in the first full cellcavity 1022 a, or the ratio may be from about 1.2 to about 1.5, such asfrom 1.3 to 1.4, for example.

FIG. 11 is a graph of energy E (arbitrary) versus Z (cm) for theaccelerator 1000. The energy is normalized to vary from 1.0 to −1.0. Z(cm) corresponds to the distance from the tapered aperture 1056. Thecell corresponding to the distance Z is also indicated. The peak energyin the buncher cell cavity 1020 a is near the anode plate 1005. The peakenergy in each full cell cavity 1022 a-1042 a is found at a distance Zfrom the tapered aperture 1056 to the center of each full cell cavity.The energy passes through zero (0) in each coupling cell 1055 and1070-1088. As shown in FIG. 11, the peak energy of the field in thefirst buncher cell cavity is about 0.7 and the peak energy of the fieldin the first full cell cavity 1022 and subsequent cell cavities 1024a-1042 a is +/−1.0. A field step ratio within these ranges has beenfound to launch the electron beam from the buncher cell cavity 1020 a tothe first full cell cavity 1022 a at an appropriate phase for theselected cell cavity length (in this example from about 0.78λ/2 to about0.82λ/2, where λ is the free space wavelength) and the overall length ofthe accelerator 1000 (14.3 cm in this example). Use of such a field stepratio α to provide simultaneous control of spectrum size and spot-sizehas in the past been accomplished by varying slot length in side coupledcavities, as in U.S. Patent Publication No. US 2005/0134203A1, forexample.

In one embodiment, the field step ratio α is controlled by the diameterof the buncher iris 1054, which acts as a coupling element for theelectromagnetic field propagating through the resonant cells 1022 a-1042a. With a buncher cell cavity 1020 a having a first diameter D1 and aniris 1054 having a second diameter D2, as indicated in FIG. 6 a, varyingthe iris diameter to a third diameter to vary the field step ratio αchanges the steady state amplitude in the buncher cell cavity 1020 a.The inner diameter of the buncher cell cavity 1020 is therefore adjustedto correct the frequency shift resulting from the change in irisdiameter. Alternatively, the field step ratio α may be controlled byintroducing such a frequency error. In this example, the buncher celliris diameter D2 is 6.52 mm and the buncher cell cavity maximum diameterD1 is 26.73 mm. The remaining cell cavities 1022 a-1040 a have maximumdiameters D4 of 27.07 mm and iris 1066 diameters D5 of 6.44 mm, asindicated in FIG. 7.

The second ratio is between the length L_(b) (see FIG. 6 a) of thebuncher cell cavity 1020 a and half the free space wavelength (λ/2),which is referred to as the buncher cell ratio. The buncher cell ratioaffects the relation between the phase of the electron beam in thebuncher cell cavity 1020 a and the phase of the field in the first fullcell cavity 1022 a, and focusing. The free space wavelength λ at 9.3 GHzis 32 mm. The buncher cell length L_(b) is in effect determined by thedepth De1 of the buncher cell cavity 1020 a, which, in this example, isless than the depth of the other cell cavities 1022 a-1040 a. This hasbeen found in simulation to facilitate the arrival of the slow movingelectrons injected into the buncher cell cavity 1020 a, into the nextcell cavity 1022 a at optimal phase. In this example, where the gunvoltage is low (29 kV-30 kV), the length L_(b) of the buncher cellcavity is about ⅔ the depth De4 of the other cell cavities 1022 a-1040a. In one embodiment, this ratio is less than one-half (½). In oneexample, the length L_(b) of the buncher cell is 4.81 mm and the bunchercell ratio is 0.3 ((4.81 mm)/(½) (32 mm)=0.3).

The third ratio is between the cell period L_(p) and half the free spacewavelength (λ/2), which is referred to as the cell period ratio L_(p)(λ/2), (where the cell period L_(p) is the distance between the centerof one accelerating cell to the center of an adjacent accelerating cell,as shown in FIG. 4). In one example, the cell period ratio L_(p)/(λ/2)is adjusted to provide a sharp spectrum with the chosen field step ratioα and the second buncher cell ratio L_(b)/(λ/2). Cell length L_(p)affects the quality Q of the accelerator body 1000. Too short a celllength L_(p) spoils the Q, resulting in increased power requirements ata given energy and requiring a more powerful modulator 114 and magnetron126, increasing the size and weight of the X-ray head 118. In thisexample, the cell period L_(p) between adjacent accelerating cavities1022 a-1040 a is less than about one-half the free space wavelength(λ/2). With this relatively short cell length, electrons traveling wellbelow the speed of light and accelerated in one cell will arrive at thenext cell in the proper phase relative to the standing electromagneticmicrowave field, for additional acceleration. The optimal availablemicrowave cell period ratio will depend on the intended range ofoperating electron gun voltage, the desired energy of the electron beam,and the microwave power provided by the magnetron 126. In this example,the cell period ratio is less than 1 and greater than 0.70. The cellperiod ratio may be about 80% of (λ/2), such as from about 0.78 to about0.82 of (λ/2), for example. The ratio is typically 1.0 in known highenergy accelerators.

The actual cell period L_(p) selected may depend on the field step ratioα. If the field step ratio is 1.3 in the accelerator 1002 of thisexample, the cell period is 12.5 mm, and the cell period ratio is 0.78((12.5 mm)/(½)(32 mm)=0.78). Adjustment of the cell length in the designfacilitates phasing of the electrons with the standing electromagneticwaves in the accelerator 1002.

The magnetron 126 is selected to drive the accelerator cavities1020-1042 at the selected frequency. The frequency of the microwaveenergy is selected such that the chain of coupled resonant cells areexcited by standing waves with less than π/2 radian phase between eachcoupling cell and adjacent accelerating or resonant cell (periodlength). In this example, the frequency is 9.3 GHz and the buncherdiameter is 26.65 mm.

To provide a smaller and lighter accelerator 1000 with a good Q, such as7700 in this example, cavity depth of the full resonant cell cavities1022 a-1040 a is increased as much as possible at the expense of iristhickness and depth. In this example, the cavity depth is 4.78 mm in thehalf period cells and 3.32 mm in the buncher cell cavity 1020 a. Theiris thickness of each half-cell is small in this example (about 1 mm).This reduces the number of accelerating cells required to accelerate theelectrons to the desired energy, and therefore the length.

As discussed above, the aperture 1056 scrapes off about half of theelectron beam as the beam is injected into the buncher cavity 1020 a.The capture fraction by the accelerator body 1002 is from about 10% toabout 15%. The resulting lower beam current lowers the powerrequirements of the accelerator 1000, facilitating the size and weightreductions discussed above, and the use of batteries 110.

Summarizing certain dimensions and characteristics of components of theaccelerator body 1002 in this example:

the accelerator body 1002 has an outer diameter of 35 mm;

the buncher cell cavity 1020 a has a maximum diameter D1 of about 26.71mm;

the buncher cell iris 1054 has a diameter D2 of about 6.52 mm;

the buncher cell cavity 1020 a has a depth De1 of 3.32 mm;

buncher cell length L_(b) is 4.81 mm;

each half-cell 1060 has an outer diameter of 35 mm;

the first full cavity 1022 has a maximum diameter D4 of about 27.07 mm(matching the maximum inner diameter of the half-cell 1060);

the first full cavity 1022 has a depth DeF (see FIG. 5) of about 9.56 mm(double the depth De4 of the half-cell 1060);

the coupling cavities 1055 and 1070-1088 have a maximum inner diameterD6 of about 26.65 mm (matching the diameter of the coupling cavities inthe half-cells 1053, 1060);

the coupling cavities have depths of about 0.98 mm (double the depthsDe3, De6 of the buncher cell 1053 and the half-cells 1060);

the iris 1064 have diameters D5 of about 6.44 mm;

the circumferential edge of the iris in this example is radiused;

the thickness De6 of the iris is about 1 mm; and

the cavities have radiused portions that are fully radiused.

As discussed above, the depth De1 of the buncher cell cavity 1020 a(3.32 mm) and the depths De4 of the following half-cell cavities 1062(4.78 mm) are different. The buncher cell cavity iris diameter D2 (6.52mm) is also different than the following half-cell iris diameters D5(6.44 mm). The smaller iris diameters D5 of the half-cells 1060 providewider modal separation. The accelerator 1000 is therefore less sensitiveto thermal effects, decreasing problems during accelerator warm up, forexample.

The Modules

The modules 102, 104, 108 protect the system components from dust, rain,and shock. The modules 102, 104, 108 may comprise stiff or flexiblematerial. Each module may include recessed handles, recessed/protectedvents, and/or recessed connectors with caps to protect contacts fromdust. In one example, the modules 102, 104, 106 comprise polymers, suchas polyurethane or glass filled polyethylene, which are durable,moldable, and lightweight. The modules 102, 104, 106 may be stackable.

As discussed above, components within a module, such as the third module106, may be coupled to a strongback of material, as discussed above withrespect to FIGS. 10 a and 10 b, to support and mechanically isolate thecomponents and protect them from physical shock, movement, falling, etc.Aluminum may be used, for example. The strong back may be directlyconnected to the case or may be coupled to the case by elastomericisolators 1250, such as springs or resilient material. In addition, thestrong-back facilitates field testing, eases maintenance, andfacilitates field replacement of the superstructure. The first andsecond modules 102, 104 may also include a strong back and elastomericisolation, if desired. The strong back may be a rigid, lightweightmaterial, such as aluminum.

Electromagnetic interference (EMI) shielding may be provided in any orall modules 102, 104, 106. EMI shielding may be provided by copper orsilver paint, for example. EMI shielding may also be provided by vacuumdeposited aluminum (VDA) on the inner and/or outer surface of themodules 102, 104, 106, for example. Integrally molded wire mesh may beprovided in the vents or other such openings, for example.

FIG. 12 is a cross-sectional view of another example of the internalconfiguration of the X-ray head 118 in the third module 106. In thisexample, the module 106 comprises a case 2010 of a lightweight,deformable material, such as plastic. An aluminum bar 2020 is attachedto an upper wall 2030 of the case 2010. The bar may be about 3 inches(76 mm) wide and about ½ inches (12.7 mm) thick, for example. Thealuminum bar 2020 may be clipped to the upper wall 2030 by clips 2035,for example. The accelerator 1000, magnetron 126, and other componentsof the X-ray head 118, are suspended from the aluminum bar 2020 by shockmounts, such as springs 2040. Double springs may be used to decreaseswaying. The accelerator 1000 and the magnetron 126 are suspended suchthat there are clearances 2050, 2060 below the accelerator 122 and themagnetron 126, which allow for movement of these components within thecase 2010 and deformation of the case 2010. A second, rigid aluminum barmay be provided between the springs and the first bar.

FIG. 13 is a cross-sectional view of another example of the internalconfiguration of the X-ray head 118 in the third module 106 that issimilar to the view of FIG. 12, except that a second aluminum bar 2021is provided, coupled to the first bar by springs 2041. The components ofthe X-ray head 118 are coupled to the second bar 2021.

Two fans 2070, 2080 are attached to the aluminum bar 2020 adjacent tothe air inlet openings through the case 2010. A flexible duct 2090extends from the fan 2070 to the magnetron 126, to provide cooling airto the magnetron. A flexible duct 2100 also extends from the fan 2070.Flexible tubes 2110 extend from the duct 2100 to various locationsaround the accelerator 1000, including to the cooling fin assemblies1150 a-1150 d, to cool the accelerator body 1002. Guides, such as guides1151 coupled to the cooling fin assemblies 1150 a-1150 d, are notprovided in this configuration.

Air outlet openings 2120, 2130 are provided for air to flow out of thecase 2000. A plastic rain cover may be provided over the openings 2075,2085 to the fans 2070, 2080. Radio-frequency interference screens may beprovided in the air openings 2120, 2130. The inner and electromagneticradio-frequency shielding, as well.

The power/control cables 130/132 may be coupled to the case 2010 via aruggedized connector, such as those provided by Caton ConnectorCorporation, Kingston, Mass.

One handle 2140 is provided in this example. The handle 2140 folds inwhen not in use.

As mentioned above, the case may also include louvered vents and/or fansfor thermal control, as well. Hot and/or cold kits could also beprovided.

The case 2000 may be used underwater by attaching a snorkel to the airopenings outlets 2120, 2130, an inlet to attach an air tank hose to eachair inlet opening 2075, 2085, for providing cooling air, and gaskets atthe case tab. High voltage hold off with moisture may be provided byadditional potting, if needed.

One of ordinary skill in the art will recognize that changes may be madeto the embodiments described above without departing from the spirit andscope of the invention, which is defined by the claims below.

1. A charged particle accelerator, comprising: a source of chargedparticles; an accelerator body comprising: a buncher cell defining abuncher cell cavity; wherein the source of charged particles is coupledto the buncher cell to inject charged particles into the buncher cellcavity; and the buncher cell cavity captures and radiofrequency (RF)focuses charged particles injected by the charged particle source, intocharged particle beam; a plurality of linearly arranged cells definingperiodic, linearly arranged accelerating cavities downstream of thebuncher cell to receive and accelerate the charged particle beam; and anoutput cell downstream of the accelerating cells to receive and outputthe accelerated charged particle beam; wherein: the buncher cell, theperiodic cells, and the output cell further define a plurality oflinearly arranged on-axis coupling cavities between respective cells;the buncher cell and a first periodic cell following the buncher cellare configured such that a field step ratio between the peak amplitudeof an electric field in a first accelerating cell cavity and the peakamplitude of an electric field in the buncher cell cavity is greaterthan one (1), during operation; and a cell period ratio between a celllength from a center of one accelerating cavity to a center of nextaccelerating cavity, and half the free space wavelength of theaccelerator body during operation, is less than one (1).
 2. Theaccelerator of claim 1, wherein the field step ratio is less than two(2).
 3. The accelerator of claim 1, comprising periodic couplingcavities between the periodic accelerating cavities.
 4. The acceleratorof claim 1, wherein the field step ratio is from 1.2 to 1.5.
 5. Theaccelerator of claim 4, wherein the field step ratio is from 1.3 to 1.4.6. The accelerator of claim 1, wherein the cell period ratio is greaterthan 0.78 and less than 0.82.
 7. The accelerator of claim 1, wherein thefield step ratio is 1.3, the cell length is 12.5 mm, and the cell periodratio is 0.78.
 8. The accelerator of claim 1, configured to define aparticle beam having a spot size encompassing 75% of the beam on thetarget having a diameter of less than 2 mm, during operation.
 9. Theaccelerator of claim 1, weighing seven pounds (3.2 kg) or less.
 10. Theaccelerator of claim 1, further comprising: a plurality of fins coupledto an exterior wall of the accelerator body.
 11. The accelerator ofclaim 10, further comprising: a cover covering at least some of theplurality of fins, wherein the cover, the fins covered by the cover, andthe exterior surface of the accelerator body proximate the covered finsdefine a cooling manifold having a first opening for air to enter thecooling manifold, and a second opening for air to exit the coolingmanifold.
 12. The accelerator of claim 1, further comprising: amagnetron coupled to the accelerator body.
 13. The accelerator of claim12, wherein the magnetron drives the accelerator body withradiofrequency power having a frequency selected to excite theaccelerating cells with standing waves with π/2 radian phase between acoupling cell and a next accelerating resonant cell, during operation.14. The accelerator of claim 13, wherein the magnetron drives theaccelerator body at a frequency of 9.3 GHz, during operation.
 15. Theaccelerator of claim 14, wherein the charged particle source comprisesan electron gun configured to operate at the same voltage as themagnetron, or lower voltage.
 16. The accelerator of claim 1, comprisingten periodic accelerator cavities between the buncher cavity and theoutput cavity.
 17. The accelerator of claim 1, wherein: the chargedparticle source comprises an electron gun comprising an anode platecoupled to the buncher cell; the buncher cell defines a half-cell; thebuncher cell cavity is defined by the half-cell and the anode plate; andthe anode plate defines an aperture having an entrance to the bunchercavity dimensioned to remove charged particles at a periphery of thecharged particle beam.
 18. The accelerator of claim 17, wherein thediameter of the entrance is dimensioned to remove at least half of thecharged particles in the beam.
 19. The accelerator of claim 1, wherein abuncher cell ratio between a length of the buncher cell and half thefree space wavelength of the accelerator is less than one-half.
 20. Theaccelerator of claim 19, wherein the buncher cell ratio is 0.3.
 21. Theaccelerator of claim 1, further comprising: a tube adjacent to an outerwall of the accelerator body, to provide cooling or heating fluidadjacent to the outer wall.
 22. The accelerator of claim 1, furthercomprising a target coupled to the output cell, wherein impact ofcharged particles on the target causes generation of radiation.
 23. Thecharged particle accelerator of claim 1, configured to generateradiation having a peak energy of about 1.0 MeV.
 24. A radiationgeneration source, comprising: an electron source; an accelerator bodycomprising: a buncher cell defining a buncher cell cavity; wherein theelectron source is coupled to the buncher cell to inject electrons intothe buncher cell cavity; and the buncher cell cavity captures andradiofrequency (RF) focuses the electrons injected by the electronsource, into an electron beam; a plurality of linearly arranged cellsdefining periodic, linearly arranged accelerating cavities downstream ofthe buncher cells to receive and accelerate the electron beam; and anoutput cell downstream of the periodic cells to receive and output theaccelerated electron beam; and a target coupled to the output cell,wherein impact of accelerated electrons on the target causes generationof X-ray radiation; wherein: the buncher cell, the periodic cells, andthe output cell further define a plurality of linearly arranged on-axiscoupling cavities between respective cells; the buncher cell and a firstperiodic cell following the buncher cell are configured such that afield step ratio between the peak amplitude of the electric field in afirst accelerating cell cavity and the peak amplitude of the electricfield in the buncher cell cavity is greater than one (1), duringoperation; and a cell period ratio between a cell length from a centerof one accelerating cavity to a center of a next accelerating cavity,and half the free space wavelength of the accelerator body duringoperation, is less than one (1).
 25. The source of claim 24, wherein thefield step ratio is less than two (2), during operation.
 26. The sourceof claim 24, wherein the output cell defines an inwardly tapered passagefrom a cavity to the target.
 27. The source of claim 24, wherein: thetarget comprises a copper substrate; and a tungsten layer coupled to thecopper substrate; wherein the thickness of the tungsten layer is lessthan 0.25 mm.
 28. The source of claim 27, wherein the thickness of thetungsten layer is less than 0.20 mm.
 29. The source of claim 28, whereinthe thickness of the tungsten layer is less than 0.10 mm.
 30. The sourceof claim 29, wherein the thickness of the tungsten layer is less than0.05 mm.
 31. The source of claim 24, wherein a buncher cell ratiobetween a length of the buncher cell and half the free space wavelengthof the accelerator body is less than one-half.
 32. The source of claim31, wherein the buncher cell ratio is 0.3.
 33. A portable radiationgeneration system, comprising: a first module comprising a power supply;and a second module separate from the first module, the second modulecomprising a charged particle accelerator; wherein: the second moduleand the first module are configured to be selectively electricallycoupled to each other to provide power from the first module to thesecond module, during operation; and the first and second modules areeach portable by hand by one or two people; the charged particleaccelerator comprising: a source of charged particles; an acceleratorbody; and a target, wherein impact of charged particles on the targetfrom the accelerator body causes generation of radiation; theaccelerator body comprising: a buncher cell defining a buncher cellcavity coupled to the source of charged particles to receive chargedparticles injected by the source; and the buncher cell cavity capturesand radiofrequency (RF) focuses injected charged particles, into anelectron beam; a plurality of linearly arranged cells defining periodic,linearly arranged accelerating cavities downstream of the buncher cellto receive and accelerate the charged particle beam; and an output celldownstream of the accelerating cells to receive and output theaccelerated charged particle beam; wherein: the buncher cell, theperiodic cells, and the output cell further define a plurality oflinearly arranged on-axis coupling cavities between respective cells;the buncher cell and a first periodic cell following the buncher cellare configured such that a field step ratio between the peak amplitudeof an electric field in a first accelerating cell cavity and the peakamplitude of an electric field in the buncher cell cavity is greaterthan one (1), during operation; and a cell period ratio between a celllength from a center of one accelerating cavity to a center of nextaccelerating cavity, and half the free space wavelength of theaccelerator during operation, is less than one (1).
 34. The portableradiation source of claim 33, wherein: the power supply comprises atleast one battery.
 35. The portable radiation source of claim 33,further comprising: a third module separate from the first and secondmodules; the third module comprising a modulator, the third module beingconfigured to be selectively coupled to the first module and to thesecond module to receive power from the first module and provide powerto the second module, during operation.
 36. The portable radiationgeneration system of claim 33, wherein the system weighs less than 300pounds (136 kg).
 37. The person portable radiation generation system ofclaim 33, wherein the second module further comprises: at least onewall; and at least one resilient member.
 38. The portable radiationgeneration system of claim 33, wherein the second module furthercomprises: at least one wall defining at least one air inlet opening; aplurality of fins coupled to an exterior surface of the acceleratorbody; a cover covering at least some of the plurality of fins, whereinthe cover, the fins covered by the cover, and the exterior surface ofthe accelerator proximate the covered fins define a cooling manifoldhaving a first opening for air to enter the cooling manifold and asecond opening for air to exit the cooling manifold; at least one fanproximate the at least one air inlet vent to move air through the firstmodule; and a guide to direct air into the first opening.
 39. Theman-portable radiation generation system of claim 38, furthercomprising: a duct to convey air from the at least one air inlet vent tothe guide.